1. Field of the Invention
The present invention relates to the field of hydrodynamic bearing assemblies of the type, which provides support and rotation for a high-speed spindle element. More specifically, the present invention relates to an improved apparatus for maintaining the stiffness of the shaft in a fluid dynamic (FDB) motor incorporating thermal compensation.
2. Background of the Invention
Disk drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm, which moves radially over the surface of the disk. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information; thus the disks must be rotationally stable.
During operation, the disks are rotated at very high speeds within an enclosed housing by means of an electric motor, which is generally located inside the hub or below the disks. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disk drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits the data track density and the overall performance of the disk drive system.
Another problem is related to the application of hard disk drives in portable computer equipment, resulting in severely increased requirements for shock resistance. Shocks create relative acceleration between the disks and the drive casing, which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long-term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disk drive industry has been to continually shrink the physical dimensions of the disk drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferromagnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the desirability of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disk housing. However, air does not provide the lubricating qualities of oil. The relatively higher viscosity of oil allows for larger bearing gaps and therefore looser tolerance standards to achieve similar dynamic performance.
A common type of fluid dynamic bearing comprises a shaft extending through the sleeve or hub with one or more radially extending plates supported from the shaft. A hydrodynamic bearing is provided between the shaft and the bore through the hub, with the fluid, which occupies the gap between the inner surface of the bore and the outer surface of the shaft providing the stiffness for the shaft. Without this stiffness, the shaft is prone to tilting or wobbling over the life of the motor. As a result, any hub or disk supported for rotation by the shaft is prone to wobbling or tilting. Any such tilting or instability in the hub or disk would make reading or writing of data on the disk surface very difficult, and diminish the life of the motor and the disk drive in which it is used.
However, the very fact that a conventional fluid dynamic bearing design relies on the use of a fluid in a very narrow gap between a shaft and surrounding bore for establishing and maintaining radial stiffness creates a problem due to the substantial range of temperatures over which the motor must operate. In known journal bearing designs for the shaft, the temperature of the fluid when the system is at rest may be about 25xc2x0 C.; in operation, the fluid temperature can be 70xc2x0 C. or more. Clearly, the viscosity of the fluid will change with the fluid becoming less dense and providing substantially less stiffness for the shaft. Thus, unless elaborate systems are incorporated into the design, it is very difficult to maintain the desired level of radial stiffness for the shaft over the entire range of operating temperatures of the disk drive.
Efforts have been made to modify the fluid used in the fluid dynamic bearing gap to minimize the changes in viscosity with changes in temperature; but such fluids can add to the cost of the bearing and motor, and have not fully achieved the goal of temperature compensation over a wide range of temperatures.
Further, adequate compensation typically cannot be provided over the operating temperature range simply by choice of the material for the shaft or the surrounding bore. If no compensation is incorporated into the fluid, then the shaft material would be required to have a high thermal expansion capability; however, both such known materials also have a low material elastic modulus, i.e., they are prone to vending and it is extremely difficult to maintain their stiffness.
Therefore, the problem remains of developing a means for maintaining the stiffness of the shaft and the journal fluid dynamic bearing, which supports it over a wide range of operating temperatures.
It is therefore an objective of the present invention to provide a hydrodynamic bearing design, which is simple and reliable in design, while incorporating means for compensating for temperature variations while maintaining the radial stiffness of the system.
It is a further objective of the invention to provide a hydrodynamic bearing design having means for substantially maintaining the stiffness of the shaft used in a single plate fluid dynamic bearing system over a wide range of temperatures.
It is a further related object of the invention to provide stiffness compensation for the shaft in a fluid dynamic bearing system, utilizing a design which is relatively inexpensive and easy to replicate in a high volume manufacturing process.
These and other objectives of the invention are achieved by providing as a part of the fluid dynamic bearing, a central shaft which is rotating inside the bore of the surrounding sleeve or hub, the shaft comprising an internal, high elastic modulus (relatively stiff) shaft/plug inside a highly thermal expansive external cylinder. This shaft assembly (cylinder and plug) typically comprises materials chosen such that the assembly is not as stiff as a single piece solid shaft made of a high modulus material; however, the assembly, especially because of the high thermal expansion external cylinder does allow for greater thermal expansion. This expansion of the outer cylinder with increases in temperature will cause the gap between the outer surface of the cylinder and the inner surface of the bore in which it rotates to be narrowed as temperature increases, compensating for the reduction in viscosity of the fluid which supports the relative rotation of the shaft and sleeve, and maintaining the stiffness of the overall journal style fluid dynamic bearing.
Other features and advantages of the invention will become apparent to a person of skill in the art who studies the following detailed description of a preferred embodiment of the apparatus for the present invention, given in conjunction with the following drawings.